When a flow control valve is arranged in a flow path, a pressure difference is introduced between the inlet side and the outlet side of the valve. As a result a force acts on the valve member in the direction of the fluid flow through the valve. Depending on the design of the valve, the force introduced by the pressure difference may push the valve member towards an open position or towards a closed position. If the force pushes the valve member towards the open position, a constant actuation force must be applied to the valve member when the valve is closed, in order to prevent that the force introduced by the pressure difference causes the valve to leak. On the other hand, if the force pushes the valve member towards the closed position, a large actuation force is required for opening the valve, because the force introduced by the pressure difference must be overcome by the actuation force.
In order to solve the problems described above, some flow control valves are balanced by interconnecting the low pressure side of the valve with a pressure balancing chamber which is in contact with a portion of the valve member. Thereby the resulting force on the valve member, due to the pressure difference, is reduced.
U.S. Pat. No. 6,568,656 discloses a flow control valve for a refrigeration system. The valve includes a body having an inlet, an outlet, a communicating passage between the inlet and outlet, the passage having a valve seat. The valve includes a piston chamber and a piston mounted in the piston chamber for movement toward and away from the valve seat. The piston includes an equalization passage, extending between an upper port communicating with the piston chamber, and a lower port. The lower port is arranged in a conical portion of the piston, along which the fluid flow through the valve takes place, when the valve is in the open position.
U.S. Pat. No. 4,911,404 discloses an expansion valve assembly for a refrigeration system. The valve assembly includes a valve body having an upper portion, an intermediate portion providing a valve chamber and a valve port, and a lower portion. A valve inlet and outlet communicate with the valve chamber on each side of the valve port. A piston assembly is mounted in the body for closing the valve port. The piston assembly includes a fluted nose portion and a bleed passage. The bleed passage is arranged in a fluted part of the nose portion.